Since helm-diff has added an ability to auto-detect the term to decide if it should output with color or not, helmfile had been defaulted to no-color.
This resoloves that, by adding a term-detection logic that is same as helm-diff.
As a part of this work, I have also implemented a new global flag `--color`, which is used for forcing color without relying on the term-detection logic implemented in helmfile or explicitly setting the HELM_DIFF_COLOR envvar. I hope it is useful for folks.
Ref https://github.com/roboll/helmfile/issues/2043
Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>